Steam-methane reforming is a process whereby methane (CH4) is converted to hydrogen (H2) by the addition of steam (H2O) and heat over a catalyst. Although the chemistry for CH4 is illustrated, other light hydrocarbons can be used as well. This process is usually accomplished in two phases. The first reaction occurs in a primary reformer where a flame is used to supply the necessary heat to convert methane to carbon monoxide and hydrogen via reaction 1:CH4+H2O→CO+3H2.  1)
The second reaction is slightly exothermic and occurs in shift reactors outside the primary reformer where carbon monoxide is further converted to carbon dioxide to liberate additional hydrogen using the water-gas shift reaction 2:CO+H2O→CO2+H2.  2)
Thus, the overall reaction is shown in reaction 3:CH4+2H2O→CO2+4H2.  3)
The primary reformer portion of the reaction, where reaction 1 occurs, typically uses burners to drive the endothermic reaction. Reaction 1 typically occurs in catalyst-filled reaction tubes heated externally by the burners.
Two kinds of primary reformers are in use. A side-fired reactor uses a plurality of burners fired against a refractory wall. Each burner is configured to disburse its heat in substantial radial-symmetry along the plane of the wall. Two mutually parallel walls are so configured and enclose a bank of vertical catalyst-containing tubes through which the steam and methane are fed. The second kind of primary reformer is a down-fired reactor. A down-fired reactor has columns of catalyst tubes interleaved by vertically down-fired burners. In this design, alternating columns of tubes and burners allow for a more modular design that can be expanded to very large sizes. For this reason, the down-fired reactor may be preferred in modern installations.
Down-fired combustion suffers from several infirmities. The first is that the fired direction is contrary to the direction of natural buoyancy. Owing to particulars of the process, the fuel pressure may be relatively low and the fuel may include impurities from a pressure-swing adsorption system often used later in the process to purify the end product. Thus, the flames are fired against the buoyant direction with low momentum. If the combustion process is not finished before the fuel and air momentum are substantially exhausted, then the flame will bend and ultimately reverse direction. Inasmuch as the catalyst tubes are in close proximity to the burners, such bending leads to flame impingement on the catalyst tube. If the flame impinges on the catalyst tube, it will generate a carbonaceous deposit on the inside tube furnace known as coke. The effect of coke deposition is to insulate the tube from the process fluid. Since the process fluid cools the tube wall, coke deposits act to insulate the tube wall, and the tube may develop hot spots as localized overheating on the fired side of the tube.
In an effort to counter the normal buoyant force with greater momentum and to reduce the flame length and increase the speed of fuel burning, sometimes a high-pressure line of refinery gas is added to supplement the main combustion of low-pressure gas. However, this adds significant expense in that an additional and independent supply of fuel gas must be plumbed, controlled, and maintained. Moreover, even with a high-pressure fuel line, flame impingement can still be a problem. Coking can be removed from the inside of a tube using special methods referred to a “de-coking.” However, the de-coking cycle can last days, during which the unit cannot produce hydrogen product.
What is needed is an effective method of flame control that does not exclusively rely on fuel momentum or require the complexity of adding high-pressure fuel gas.